1. Field of the Invention
The production of free-standing articles by chemical vapor deposition (CVD) can provide such articles with enhanced properties. The use of CVD to provide articles in near-net shape (NNS), such that only minimal finishing is required to provide the article in its finished shape, is especially useful for producing articles having critical dimensional tolerances from materials, such as silicon carbide, which are difficult to shape by conventional machine shaping techniques. The present invention provides an improved method of mounting substrates during the CVD process. It is particularly applicable to the production of near-net shape silicon carbide articles.
2. Description of Related Art
The advantages of silicon carbide as a fabrication material for astronomical X-ray telescopes and the experimental use of small scale CVD processing to prepare conical silicon carbide shells were recently described by Geril et al. in xe2x80x9cThin Shell Replication of Grazing Incident (Wolter Type I) SiC Mirrorsxe2x80x9d, SPIE Proc., 2478, 215 (1995).
Free-standing silicon carbide materials produced by CVD processing in applications requiring a high degree of surface smoothness and polishability are described in U.S. Pat. No. 5,374,412. Apparatus and process conditions used to produce such articles are described in that patent. U.S. Pat. Nos. 4,990,374; 4,997,678 and 5,071,596 further describe CVD processes for producing free-standing silicon carbide materials by the pyrolytic deposit of SiC on a mandrel.
Typically, CVD derived articles are produced by CVD deposit of the desired material on a substrate, followed by separation of the article from the substrate. One prior method produces a relatively large sheet of monolithic SiC on a flat graphite mandrel coated with a thin layer of a release coating. Pyrolysis of methyltrichlorosilane in argon and excess hydrogen produced a deposit which, after separation from the mandrel, was cut into multiple susceptor rings for use supporting wafers in semiconductor processing furnaces. While this method produces satisfactory parts, they are not produced in near-net shape and require substantial machining. The production of the deposit in near-net shape is desirable to reduce the amount of waste material generated and reduce the amount of machining required.
Several methods of controlling or isolating the deposit of silicon carbide to one intended side of the substrate during chemical vapor deposition are described in U.S. Pat. Nos. 4,963,393 and 4,990,374. In U.S. Pat. No. 4,963,393, a curtain of flexible graphite cloth is arranged to shield the backside of the substrate from the flowing reacted precursor gases, whereby silicon carbide deposits on the backside of the substrate are avoided. In U.S. Pat. No. 4,990,374 a counterflow of a non-reactive gas flows from behind the substrate past the substrate""s peripheral edge whereby the reactive deposition gases and the deposit they produce are confined to the front face of the substrate.
Another prior technique controls the deposition by providing a channel surrounding that portion or zone of the substrate surface where the deposit is desired. The channel functions to restrict flow of the reactive deposition gases to the substrate surfaces surrounding the deposition zone whereby any deposit on the surrounding surfaces is substantially thinner than the deposit formed in the deposition zone.
Still another previous method provided multiple shaped graphite ring mandrels (substrates) mounted along the extent of the deposition chamber by detachable graphite mounts gripping the edge of the rings. Silicon carbide was deposited on both sides of the mandrels, the mandrels removed from the deposition chamber and the edges of the deposits on the mandrels machined to release the bottom and top deposits as two separate silicon carbide articles. This process resulted in relatively heavy deposits of silicon carbide bridging the graphite mandrel and the graphite mounts, necessitating difficult machining in the vicinity of the areas occupied by the mounts, and often resulted in cracks developing in the deposits during separation of the mount from the mandrel. These cracks often propagated through the desired product causing it to be rejected. If sufficient mounts are not used, the increased weight of the deposit on the mandrels sometimes caused the mandrels to slip from the mounts damaging the deposits and adjacent mandrels.
Chemical vapor deposition (CVD) has been used to produce both free-standing articles and coatings of various materials, such as, silicon carbide. Typically, such a process involves reacting vaporized or gaseous chemical precursors in the vicinity of a substrate to result in silicon carbide depositing on the substrate. The deposition reaction is continued until the deposit reaches the desired thickness. If a coated article is desired, the substrate is the article to be coated and the coating is relatively thin, generally less than 100 microns (0.1 mm) thick. If a free-standing article or silicon carbide bulk material is desired, a thicker deposit, generally greater than 0.1 mm thick, is formed as a shell on the substrate and then separated from the substrate to provide the silicon carbide article.
In a typical silicon carbide bulk material production run, silicon carbide precursor gases or vapors are fed to a deposition chamber where they are heated to a temperature at which they react producing silicon carbide. The silicon carbide deposits as a shell on a solid substrate. The deposition is continued until the desired thickness of silicon carbide is deposited. The substrate is then removed from the deposition chamber and the shell separated therefrom. Monolithic silicon carbide plates and cylinders have been produced by applying such chemical vapor deposition (CVD) techniques on suitably shaped substrates. Some articles require a deposit about one inch thick, which can require deposition processing extending three hundred hours or longer.
Once the silicon carbide precursor gases or vapors are brought to the appropriate conditions to cause them to react, they produce silicon carbide which deposits on any available surface. Such deposit generally is not limited to the intended surface of the substrate and generally extends past such surface to adjoining surfaces as well as depositing on the walls, housing and any other available surfaces associated with the deposition chamber. In prior processes, the silicon carbide deposit has extended past the dimensional limits of the substrate covering adjacent portions of the support structure holding or clamping the mandrel/substrate in position in the deposition chamber. These extraneous deposits not only consume the deposition chemicals, they can be relatively thick requiring either their removal from the production equipment or that the equipment be routinely replaced. The consumption of deposition chemicals and refurbishing or replacement of furnace equipment add considerable expense to the cost of the process. Moreover, it is generally necessary to fracture the deposits to remove the substrate from their mount in the deposition chamber. Fracturing of the relatively thick deposit often results in the formation of cracks which propagate through the deposit. Such cracks are not acceptable in most of the intended applications of the silicon carbide articles, and result in the article being rejected. The prevalence of propagated cracks in relatively thick chemical vapor deposits of silicon carbide have limited the size of articles which can be produced commercially by this method. Moreover, recognition of the potential capacity of CVD silicon carbide deposits to bridge joints between adjacent stacked substrates and the subsequent difficulty of separating and removing individual substrates from such a stack has precluded the use of stacked multiple substrates in the commercial production of silicon carbide articles.
Some previous techniques have sought to reduce the above noted extraneous deposits by controlling the flow of the reactive deposition gases so that they contact and form the intended deposit on only one side of the substrate. If the thermal properties of the substrate used in such processes are not perfectly matched with those of the deposit, stresses are introduced during the cool-down from the deposition temperature which cause distortion of the deposit, and, in extreme cases, can cause cracks to develop in the deposit. Moreover, these limitations restrict the amount of material which can be produced in a given size deposition chamber.
The present invention is directed to a process, and associated apparatus, for producing bulk materials by chemical vapor deposition wherein extraneous deposits (i.e. deposits formed on deposition chamber surfaces other than the intended production surface of the substrate) are minimized. Reduction of such extraneous deposits provides economic benefits both with respect to the cost of raw materials and the labor costs associated with CVD processing. The present invention further provides a relatively inexpensive method of mounting the substrate in the deposition chamber whereby the substrate can be readily separated from the mounting structure without causing cracks to propagate through the deposited bulk material. A further advantage of the inventive process is the use of devices to mount the substrate which are sufficiently inexpensive to be disposable, whereby the removal of extraneous material deposited thereon is not necessary. The inventive process can be used to produce articles of silicon carbide or of any of the materials which are capable of being produced by CVD processing, such as zinc sulfide, zinc selenide, boron nitride, boron carbide, silicon nitride, titanium diboride and aluminum nitride.
According to the present process, suitably shaped substrates, or mandrels, are suspended and solely supported in the deposition chamber by linear suspension supports, preferably flexible linear suspension supports, such as ropes or cables, which engage the substrate(s) at their edge(s) and extend to supporting structure on, or in, the deposition chamber housing. The suspension supports are referred to as linear because they have a longitudinal dimension which is much greater than their transverse dimension which is located adjacent the substrate. The linear suspension supports and the substrates are designed to enable minimal contact between the surface of the substrate and any other apparatus in the deposition zone, while providing adequate support to assure that the substrate does not break loose as it gains weight during the deposition process. While some deposition occurs on the linear suspension supports during the process, the minimal contact of the support with the substrate provides a degree of flexibility which enables the separation of the support from the coated substrate without causing cracks to propagate throughout the deposit. Moreover, any deposits formed on the preferred rope or cable linear supports can often be removed by merely flexing the rope or cable. Should the linear suspension supports become unduly coated or contaminated in some other manner, they are relatively inexpensive to replace, and therefore do not require extensive cleaning or decontamination procedures to enable them to be reused.